mucking hand

[dark-o]

player A is betting on the river, and player B is paying, then player A muck his cards. here is the question:
1. what is the rule about mucking hand(player A should first show the cards)?
2. now MUST player B show his cards(to win the pot) while player A mucked?

Can player muck his cards anytime hi like or he MUST show his cards? is there any rule where player MUST show his cards even if he is not claiming the pot?

P.S.
sorry for bad english